In this Letter we analyze the energy distribution evolution of test
particles injected in three dimensional (3D) magnetohydrodynamic (MHD)
simulations of different magnetic reconnection configurations. When
considering a single Sweet-Parker topology, the particles accelerate
predominantly through a first-order Fermi process, as predicted in and
demonstrated numerically in . When turbulence is included within the
current sheet, the acceleration rate is highly enhanced, because
reconnection becomes fast and independent of resistivity and allows the
formation of a thick volume filled with multiple simultaneously
reconnecting magnetic fluxes. Charged particles trapped within this
volume suffer several head-on scatterings with the contracting magnetic
fluctuations, which significantly increase the acceleration rate and
results in a first-order Fermi process. For comparison, we also tested
acceleration in MHD turbulence, where particles suffer collisions with
approaching and receding magnetic irregularities, resulting in a reduced
acceleration rate. We argue that the dominant acceleration mechanism
approaches a second order Fermi process in this case.